1. Field of the Invention
The present invention relates to a surface treatment method for a nitride crystal used for a substrate for example of such a semiconductor device as light-emitting device, electronic device and semiconductor sensor. Further, the invention relates to a nitride crystal substrate obtained using the surface treatment method for the nitride crystal and relates to a semiconductor device including the nitride crystal substrate and a method of manufacturing the semiconductor device.
2. Description of the Background Art
A nitride crystal, particularly a group III nitride crystal such as GaN crystal and AlN crystal is very useful as a material for forming a substrate of a semiconductor device such as light-emitting device, electronic device and semiconductor sensor. Here, the nitride crystal refers to a crystal made of a nitride and includes, as a typical one, a group III nitride crystal. The group III nitride crystal refers to a crystal made of a group III element and nitrogen and includes, for example, GaxAlyIn1-x-yN crystal (0≦x, 0≦y, x+y≦1).
A nitride crystal substrate used as a substrate of a semiconductor device is obtained by shaping the outer periphery of a nitride crystal and thereafter slicing the crystal to obtain a piece of the crystal having a predetermined thickness, and grinding or lapping the surface. However, the slicing, grinding or lapping causes a surface region of the nitride crystal to have a thick work-affected layer (referring to a layer having a disordered crystal lattice formed in the surface region of the crystal, due to grinding or polishing of the crystal surface, which is applied as well in the following description), or the nitride crystal has an increased surface roughness. As the thickness of the work-affected layer of the nitride crystal substrate is larger and as the surface roughness is larger, the quality of the substrate surface is deteriorated. Further, the surface of a group III nitride crystal layer epitaxially grown on the nitride crystal surface has a large roughness and the crystallinity is deteriorated. Therefore, a semiconductor device of good quality cannot be formed.
Accordingly, as a method of forming a nitride crystal substrate from a nitride crystal, it has been done widely to slice a nitride crystal into a piece having a predetermined thickness, grind or lap the surface and further perform dry etching (see for example Japanese Patent Laying-Open No. 2001-322899) or chemical mechanical polishing (hereinafter CMP) of the surface (see for example U.S. Pat. Nos. 6,596,079 and 6,488,767), thereby removing the work-affected layer and further reducing the surface roughness.
Although the method of dry-etching the surface of the group III nitride crystal substrate can remove the work-affected layer, it is difficult to further reduce the surface roughness.
Moreover, the conventional CMP polishes the surface of the nitride crystal by supplying, to a polishing pad, a slurry including abrasive grains of a lower hardness than that of the nitride crystal to be polished, while pressing the nitride crystal against the polishing pad. However, since the nitride crystal is hard and relatively slow in response, the conventional CMP is considerably low in polishing speed and efficiency.